Method of producing continuous metallic filaments



March 28, 1961 R. B. POND 2,976,590

METHOD OF PRODUCING CONTINUOUS METALLIC FILAMENTS Filed Feb. 2, 1959 V l F \l V v 1 u v 39 2 3 United States Patent METHOD OF PRODUCING CONTINUOUS METALLIC FILAMENTS Robert B. Pond, Westminster, Md., assignor to Marvalaud, Inc., a corporation of Maryland Filed Feb. 2, 1959, Ser. No. 791,003 3 Claims. (Cl. 22-2001) The present invention relates to the production of continuous metallic filaments by extrusion and to the control of the various factors affecting the shape and size of the filaments when cast in a chamber containing a cooling medium.

In my prior Patent No. 2,825,108, issued March 4, 1958, there is described a process and apparatus for casting fine metallic filaments. According to the invention described therein fine filaments are formed by extruding molten metal through an orifice against a rotating chill surface where the molten metal is solidified into a filament. The present invention relates to a casting process wherein the molten metal is not confined by a mold on any surfaces thereof but is free cast in the presence of a cooling medium The present invention is a continuation-in-part of my earlier application Serial No. 563,615, filed February 6, 1956, now abandoned.

According to the present invention molten metal is extruded through an orifice into a chamber containing a cooling medium. The molten stream of metal is solidified into a continuous filament in the chamber. There are a number of variables in this process which must be accurately controlled in order to produce a filament of desiredshape and size. The present invention relates to the determination of these various factors and their effect upon the filament as formed. These factors include, the type of metal used, the temperature of the molten metal, the extrusion orifice dimension, the ejection velocity, the temperature and type of cooling medium used. According to the present invention by controlling each of these various factors within certain predetermined limits the size and shape of the filaments formed may be controlled.

An object of the present invention is to provide a process and apparatus for casting fine metallic filaments in the presence of a cooling medium.

Another object of the present invention is to determine the operating limits of the variable factors controlling the free casting of metallic-filaments.

Still another object of the invention is to define by a formula the relationship between the variable factors determining the shape and size of the filaments formed by a free casting process.

These and other objects and advantages of this invention will be apparent from the following description and accompanying drawing in which the sole figure of the drawing illustrates the apparatus, partly in vertical section, for practicing the method of the present invention.

Generally, the invention relates to the manufacture of continuous metallic filaments by extruding a streamline flow of molten metal into a gaseous medium through which it travels, in suspended relationship and under the force of gravity, for a distance sufiicient to effect solidification. The material employed, the ejection velocity or the pressure under which the molten metal is extruded, and the rate of cooling the extruded stream are factors which determine the distance of travel before complete solidification of the filament results, while the configuration of the extrusion orifice dictates the cross-sectional Patented Mar. 28, 196i the extruded metal stream besides shielding the same with a protective and inert fluid envelope.

With reference to the drawing, there is disclosed an elongated tubular chamber 9 supported, adjacent its upper end, by brackets 11 and a movable framework 13, shown in part. The uppermost end of the chamber 9 is flared to provide an annular flange 15 upon which the tank 17 rests and is suitably fixed, while its lower end carries an exhaust hood, 19, described hereinafter in greater detail. The chamber 9, which may be either circular or polygonal in cross-section, is provided with channel-shaped ribs or projections 21 arranged in concentric relationship with the main body of the chamber 9 and at spaced intervals longitudinally thereof. As shown on the drawing, the bottom walls 23 of the ribs 21 are substantially horizontal to provide shelves or ledges upon which cakes of solid carbon dioxide may be placed, while the remaining portion of each of the ribs can be of any suitable and convenient shape. The chamber 9 serves both to enclose the cooling gas and to maintain the same substantially quiescent and is preferably of sec tional construction to permit the chamber length to be varied for purposes described hereinafter.

The filament forming material, in a molten condition, is introduced into the tank 17 through the conduit 25 which is connected at its opposite end to a conventional melting pot or other similar structure, not shown. To prevent cooling of the molten metal, the tank 17 may be thermally insulated and in addition he provided with heating elements, not shown; along the tank wall or immersed within the molten metal itself, the upper level of which is indicated by the broken line 27. The bottom wall 29 of the tank 17 is inclined or conical in shape to prevent the occurrence of stagnant flow areas and to direct the molten metal to at least one extrusion nozzle 31 which is removably mounted centrally of the bottom wall 29. The design of the nozzle orifice will, of course, depend upon the cross-sectional configuration desired, so that a round wire can be formed by merely employing a nozzle having a circular orifice while a nozzle with a complex orifice will produce finished wires of non-circular crosssection. four-leaf clover causes the stream of extruded metal to assume a form which, when altered by the surface tension of the molten metal as the stream passes through the chamber 9, results in a finished wire of rectangular cross-section.

To insure filament continuity, the molten metal is ex truded through the nozzle orifice in a streamline flow by means of pressure, preferably supplied by a gas introduced into the tank through a separate conduit 33 leading from a supply source, not shown. The pressure required will vary with the orifice design and thus it is merely necessary to adjust the pressure until the desired flow is obtained. For-visual indication of the pressure withinthe tank 17, a conventional pressure gauge 35 is provided.

Once extruded through the nozzle orifice, the stream of molten metal is forced through the elongated chamber 9 which is filled with cool air or an inert gas, such a helium or argon. This gas is supplied from a suitable source, not shown, through the pipes 37, each having a regulating valve 39. If desired, the cooling gas can be an envelope of carbon dioxide supplied by cakes of solid carbon dioxide. In addition to providing a sharp thermal gradient externally of the nozzle 31, the use of gases other than air serve to protect the partially formed filament from oxidation.

As the molten metal is extruded through the nozzle For example, utilizing an orifice resembling a tight cover for the bin 41, yet allows the bin to be easily removed from its position illustrated by merely elevating the side 43 which is hinged at 45 to the remainder of the hood structure. By means of the exhaust ducts 47, gases can be expelled from beneath the hood and carried to a cooling chamber (not shown) for recirculation through the pipes 37, along with the similarly rechilled gases withdrawn from the upper portion of the chamber 9 through the exhaust ducts 49, To prevent damage to the exhaust hood 19, a stop 51 is fixed to floor 53 to limit the bin movement.

In practicing the teachings of the present invention, molten metal is delivered into the tank 17 through the conduit 25 while gas under pressure is introduced through the conduit 33. As heretofore mentioned, the amount of pressure imposed over the liquid metal will depend upon the particular design of the orifice. The continuous stream of molten metal extruded through the nozzle 31 passes into the chamber 9 which is continuously supplied with gas by the pipes 37, or cakes of solid carbon dioxide placed on the shelves 23. Preferably, the chamber is flushed with these cooling and protective gases before any extrusion takes place to insure that the proper atmosphere exists within the chamber 9.. As the stream of molten: metal descends through the chamber 9, the surface tension forces of the molten metal alter the shape of the molten stream into the desired configuration. In addition, the ejection of, the metal in a downward direction enables the force of gravityto. assist in main.- taining the streamline flow of the molten metal. Although. the cooling gas is continuously and uniformly circulated, its movement is not sufiicient to break the filament continuity.

The distance of travel of the filament through the chamber 9 must be such as to insure that, the filament has solidified before entering into the bin 41, and therefore depends upon such factors as the material extruded, the temperature and position of the cooling medium relative to the extruded filament, the temperature oftthe molten metal, and the extrusion pressure, Once. the necessary heightof the chamber has been determined, as for example by simple trials of the extrusion nozzle at different heights, tubular sectionsv may be added or removed from the chamber 9, as required, and the framework 13 may be elevated to position the exhaust hood 19 at the desired location above the bin 41.

Throughout the extrusion process, the gases are withdrawn from the chamber 9. and from beneath the hood 19 by means of the ducts 47 and 49 to permit non-turbulent cooled gases to contact with the continuous molten stream. The production remains continuous as long as the molten metal is supplied to the tank 17 and the cooling gases are circulated, thus the operator needs only tocheck the pressure reading of the gauge 35 and supply empty bins 41 as needed.

The invention described above is adapted for use with most all metals such as tin, bismuth, antimony, indium, aluminum, magnesium, copper and silver, with the exception of those metals having a high vapor pressure which are formed as described in my prior patent entitled Production of Filaments of High Vapor Pressure Materials No. 2,907,082, issued October 6, 1959.

Typical examples of conditions found tobe satisfactory for producing continuous metallic filaments are set forth in the following table in which, V is the ejection or extrusion velocity at the orifice necessary to achieve streamline fiow, T is the temperature of the liquid metal, T, is the temperature inf-solidification, T is .theiemperature of the gaseous atmosphere, T T, is the super heat in degrees centigrade, and T -T the heat drop also in degrees centigrade.

Orifice Height, Metal V cur/sec. Tin-Ts Diameter, Tar-TO Gas inches inches/cm.

Sn 183 5 002-, 005 212 Air 5l. 0 Sn 183 5 01th 025 212 Air 33 Sn 183 002-. 005 307 Air 5-1. 0 Sn 100 100 030-. 076 307 Air 30 The filaments produced according to the present invention are free from crystalline deformation or reorientation. Further, the filaments can be formed in various cross-sectional shapes and sizes some as small as 4 to 8 ten-thousandths of an inch in diameter, as well as larger diameters as for example A; of an inch.

DISCUSSION OF VARIABLES Since this process is one of solidification it is a process involving heat transfer, for all of the latent heat of fusion as well as the superheat (or that heat absorbed after the metal becomes molten) must be transferred from the volume of metal before the liquid-solid transformation is complete. This amount of heat is a function of the physical constants of the metal, the temperature of the molten metal, and the orifice size and can be considered in two parts, H the heat of fusion and H the supertreat.

For a unit length of molten fiber of circularcrosssection D=Diameter of the fiber (orifice diameter d=density of the metal where H =Specific heat of the metal T =Ternperature of the metal T Temtperature of solidification Thetotal-heat to be removed This heat will be removed principallyby means of the gas and. therefore thetherrnal properties ofthe gas must be evaluated. The ability of the gas to transport heat is given by its heat transfer coeificient, h. This coelfic-ient is difiicult to determine unless the mechanics of the heat flow process can be approximated and this would be difiicult where the fiber diameter is very small and the flow of the gas is streamline and axial relative to the fiber.

Despite the method used to determine the heattransfer coefiicient it will be expressed in units similar to those of the coeificient of thermal conductivity, i.e.

h=B.t.u./hr./ft. F.

Therefore the total energy which can be transferred from the fiber per second (7l T0)(llD) -5 B. t -u-jsec. where:

T =Temperature of the gas; h= heat transfer coefiicie'nt. D aDiameter, of the fiber.

aweeeo Therefore the total heat to be removed divided by the rate of heat removal persecond will result in the time required for solidification, t

S=distance from the orifice to the liquid-solid interface :V =ejection velocity Obviously, if the time required for solidification becomes appreciable then the effect of gravity on the position of the liquid-solid interface must be considered and the equation becomes I o 1+ 8 where g=gravitational constant T Generally the effect of surface tension has begun to .manifest itself by necking the fiber before such gravity corrections need be made. 7 The manipulation of the equations can be followed in this example: Compute the distance of the liquid-solid interface from the orifice when the orifice size is .010 inch dia. ejection velocity 20 feet per second and the metal is tin with no superheat. The latent heat of fusion for tin :is 26.1 B.t.u./#, the density is .264 #/in. and the melting point is 450 F. Assume the empirically determined .value .2, for the heat transfer coefiicient for air to be .correct. The value .083 is a conversion factor for converting inches to feet.

=.0695 second =1.39= feet (A) Efiect of diameter The time required for solidification and therefore S will vary directly as the diameter of the orifice, viz., it .will take a .010 inch fiber of a given metal ten times as long -to solidify as a .001 inch fiber of the same metal all .'other variables being held the same. The practical Work- *ing limits of orifice diameter are .0005 to .0395 inch. Various examples of forming filaments using difierent orifice sizes are given hereinafter.

. (B) Efiect of superheat V specific heat of a metal is generally a small per is advantageous to keep V as small as possible.

centage of the latent heat of fusion as can be seen in the table below.

Metal: Percent H is of H, Al a .23 Sb .125 Cu I .178 -Pb .48 Mg .28 Ag .20 Su .37 Zn .374

The superheat term of the equation is H (T -T and therefore for aluminum it would take approximately four degrees Farenheit of superheat before the total heat to be removed would be raised 1% In accordance with the formula the variable which is being increased is T and since it also exists in the denominator the value of 1 will not be increased linearly with the percentage increase of H H The effect can be appreciated by solving the above problem with 50 degrees of superheat.

s= (20) (.067) 1.34 feet numerator to pass to its new value but keep the value of the T in the denominator at the solidification temperature ofthe metal (T Such a treatment would cause S in the previousproblem to become 1.52 feet. This evaluation of the denominator as h(T -T is probably correct since the surface temperature'of the fiber can be considered to drop instantly to the solidification temperature and remain there until the residual melt heat is removed. It has been found that filaments can be formed with an apparatus which is commercially feasible with a superheat range of from O to degrees F. in connection with alloys it is to be understood that by superheat is meant degrees over the liquidus line.

(C) Effect of change in ambient temperature (T If the temperature of the quiet gas is allowed to increase during a run then the term T T becomes smaller as does h. Assume that in the previous problem the superheat is 50 degrees and the value of T T., is allowed to decrease to 10 degrees and there is no accompanying decrease in h, then t will become 2 /2 seconds or the distance S will become fifty feet. Such a 1 is not practical for within such time periods the surface tension effects will become pronounced and the acceleration of the column due to gravitational forces will also present problems. The value of 8:50 feetis generally untenable in that such values would cause the furnace and apurtenances to be positioned 50 feet higher than any floor or collection device. If the maximum operating height was selected to be 10 feet (with all other variables held constant then the time 1 could not be in excess of .5 second or the gas temperature should not be allowed to exceed 400 degrees. For practical purposes the ambient temperature should not be greater than /2 the melting point of the metal in degrees absolute.

(D) Efiect of change in V Obviously, the distance 8 is a direct function of V and since it is desirous to keep S as small as possible it Yet if V is maintained at too low a level, a discontinuous stream will result (as previously described) and the pro- I value is'selected by maintaining the lowest superheat, lowest ambient temperature, using a gas with the highest h and ejecting the metal with the highest V possible Without deleterious effects such as causing an intolerable turbulence in the cooling gas or impinging on a solid prior to solidification. It has been found that about 20 feet per second constitutes a maximum'ejection velocity.

(5.) Variation of V T and orifice geometry It is virtually impossible to cause 1D, T or k to change appreciably over short intervals of time (note that a change which could be reflected in the finished product over long distances is quite possible, i.e. with V =20 ft./sec., a variation of .001 in. in D occurring over a time period of one hour of continuous operation would be evidenced by a fiber diameter change of .001 in. over 72,000 feet) whereas changes which would be evidenced in the fiber at .1 inch intervals must occur within 420 micro-seconds or at a frequency of 2400 per second.

It is possible to change V with a high frequency either by mechanically or pneumatically pulsing; the liquid metal bath so that there is a repeating instantaneous change in the hydrostatic pressure at the orifice. When the apparatus is properly operated with the exception of this variable V 3. fiber results which contains more metal at one interval than at the adjacent interval. By varying the amplitude and frequency of the hydrostatic pressure change, fibers can be produced having varying size and distribution of nodules.

It is virtually impossible to cause T to change appreciably over short intervals of time but it is possible to cause T to change over that interval of space S wherein solidification occurs. materials wherein the rate of nucleation for solidification is low, it is possible to operate at selected V values with a finite established gradient of T along the axis of the metal flow so that crystal growth of single nucleus is maintained and the resultant fiber consists of a Ba mboo structure of individual grains, or long single crystal segments.

Every case previously discussed has considered the orifice as being round and a resultant round fiber. This symmetrical case is the simplest not only from the standpoint of heat transfer but also because the circular crosssection represents the lowest surface area to volume ratio or the lowest energy configuration for a continuous material. It is possible to manufacture orifices of varying shape. For purposes of illustration an orifice of a three point clover leaf cross-section will be considered. When the continuous stream of molten metal is ejected from this orifice its cross-section will be identical with that of the orifice. As soon as the metal leaves the orifice, surface tension forces Will be endeavoring to cause the clover leaf to collapse into a circular pattern. The rapidity with which the accomplishment of this new shape can occur will be a function of the surface tension and the viscosity of the metal both variables being dependent upon the temperature of the metal.

If it is desired to capture the original cross-sectional geometry of the metal steam as a final solid fiber geometry then the t value must be caused to be much less than the time required for transition from the shaped to the round stream of metal, t Since the transition of shapes is not cataclysmic the closer t approaches t the more nearl round the fiber will be. If t becomes equal to or greater than I the resultant fiber will be round. It is extremely difficult to find the exact value of 28 since a thermal analysis of this problem is, at this time, impossible and since experimental endeavors to measure the value have necessitated changing the variables of the process to cause to be less than 1 thereby probably changing the original values of b That a t -does exist and that shaped For fine fibers of relatively pure fibers can be made has been proven as can be seen by examination of the following table.

All underlined distances illustrate those conditions wherein n becomes less than tr or those cases where a shaped fiber is captured in the solid state.

Structure of "free cast fibers Fibers of this class have been produced in diameters ranging from .0005 to .034 inch and in continuous lengths. (At times mechanical means such as intermittent air blasts normal to the flow of the metal are employed for producing discontinuous lengths.) Fibers of this class have been produced for most of the commercial metals and alloys of the non-refractory class. Where alloys are free cast into fibers the melting point becomes a melting range and the lowest temperature of this range must be considered as the melting point in most of the computations. When an alloy constituent exists above its boiling point the quiet gas chamber must be pressurized to prevent disintegration of the stream by vaporization.

In the lower diameter regions (.003 to .015 inch) where the process can be most carefully controlled, ideal products have been examined for variation in diameter over several thousand feet of the product and the variation in diameter over several thousand feet of the product and the variation found to be less than 2%.

Examples of aluminum filaments formed with varying sized orifices and varying amounts of superheat are given 7 below:

Orifice Superheat, tr V1 S in .F. inches 18 "I 61 240*lsec. 2 so .107 31 .113 '34 13 6%? ii so .6535 17 100 .0565 .14 .3 as 9 With tin filaments have been formed using an orifice of .00051 inch atbetween 0 and 10 F. superheat at an extrusion velocity of 10 ft. per second. The liquid-solid interface was less than 1 inch from the extrusion nozzle. An irregular wire has been formed with aluminum 'with an orifice of .0395" wherein the liquid-solid interface was approximately 40" from the extrusion nozzle.

It can thus be appreciated that according to the'prescnt invention the relationship between the variable factors such as orifice size, .metal temperature, cooling .gas temperature, and extrusion velocity, has been clearly defined. In addition there have been set forth critical limits for these variables so that when operating within these limits fine metallic filaments of the character described can be formed.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. What is claimed as new and desired to be secured by Letters Patent is:

1. A method of forming metallic filaments of a specific cross-sectional configuration comprising the steps of heating the metal to a range of from to 100 degrees F. superheat, extruding the molten metal through a nozzle having a cross-sectional configuration identical to the desired filament cross-sectional configuration, said nozzle having an orifice size of from .0005 to .030 inch at a velocity sufiicient to maintain a continuous stream up to a maximum of 20' feet per second, passing the molten stream through a quiescent gaseous atmosphere in a chamber completely enclosing the nozzle, maintaining the gaseous atmosphere at a temperature not greater than one half the melting point of the metal, solidifying the molten metal While in a continuous stream Within 40 inches of the extrusion nozzle prior to distortion of the cross-sectional shape of molten metal stream by surface tension from the cross-sectional configuration of the nozzle, the environment to the molten metal stream being maintained under completely controlled conditions from the point of departure through said nozzle until solidified into a continuous filament.

2. A method of forming metallic filaments of predetermined non-circular cross-sectional shape having a diameter Within the range of .0005 to .034 inch comprising extruding molten metal through a nozzle having the preetermined cross-sectional shape at a velocity up to 20 feet per second sufiicient to maintain a continuous stream, passing the molten stream through a quiescent gaseous atmosphere in a chamber completely enclosing the nomle,

maintaining the gaseous atmosphere at a temperature not greater than one half the melting point of the metal,

solidifying the molten metal While in a continuous stream within the chamber into a continuous filament, the solidification distance through which the molten metal passes being less than the distance through which the molten metal would pass in being transformed by surface tension into a substantially round filament.

3. A method of forming metallic filaments having a diameter within the range of .0005 to .034 inch comprising extruding molten metal through a nozzle at a velocity up to 20 feet per second and sufficient to maintain a continuous stream, passing the molten stream through a quiescent gaseous atmosphere in a chamber completely enclosing the nozzle, maintaining the gaseous atmosphere at a temperature not greater than one half the melting point of the metal, solidifying the molten metal while in a continuous stream Within the chamber into a continuous filament, the environment of the molten metal stream being maintained under completely controlled conditions from the point of departure through said nozzle until solidified into a continuous filament, the length of time required for solidification being determined by the formula 1 the temperature of the atmosphere and h is the heat transfer coeflicient of the atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS Small Aug. 15, 1882 Horton et al. July 13, 1926 

